1. Field of the Invention
The present invention relates to a light emitting device, and more particularly, a light emitting device having a plurality of light emitting cells.
2. Discussion of the Background
A light emitting diode, which is an electroluminescence semiconductor device, emits light by recombining electrons with holes. Light emitting diodes have been prevalently used as display devices and backlights. Light emitting diode display devices may have smaller power consumption and longer lifespan than incandescent lamps or fluorescent lamps, such that light emitting diodes may replace incandescent lamps and fluorescent lamps and expand its use field to general lighting. An attempt to improve the light emitting output per chip area of a high-voltage light emitting diode and an attempt to improve reliability of the light emitting diode have been continuously conducted.
Generally, a light emitting diode may be driven and emit light when a forward current flows through it, such that the light emitting diode may be repeatedly turned on and off according to the direction of current under an alternating current (AC) power supply. Therefore, when the light emitting diode is directly connected to the AC power supply, it may not continuously emit light and may be easily damaged by reverse current.
In order to solve this problem, a light emitting diode directly connected to a high-voltage AC power supply is disclosed in U.S. Pat. No. 7,417,259, issued to Sakai, et. al. In addition to this, light emitting diodes having various structures have been developed.
According to U.S. Pat. No. 7,417,259, the light emitting diode arrays are formed by two-dimensionally connecting the light emitting diodes in series using metal wirings on an insulating substrate, such as a sapphire substrate. The two light emitting diode arrays are connected on the substrate in anti-parallel, such that they continuously emit light when current is applied from an AC power source.
Meanwhile, according to U.S. Pat. No. 7,417,259, one array is driven for a half period of the AC power supply and the other is driven for next half period thereof. That is, half the light emitting cells in a light emitting diode are driven while a phase of the AC power supply is changed. Therefore, the utilization of the light emitting cells does not exceed 50%.
Meanwhile, a light emitting diode configured by disposing an array of light emitting cells connected in series between two nodes of a bridge rectifier made by using light emitting cells on the substrate and driven by an AC power supply is disclosed in U.S. Patent Application Publication No. 2008/0017871, applied for by Lee, et al. According to U.S. Patent Application Publication No. 1008/0017871, the light emitting cell array connected to the bridge rectifier performs full-wave light emission regardless of the phase of the AC power supply, thereby making it possible to increase the utilization of the light emitting cells.
However, when the number of light emitting cells connected to the bridge rectifier is increased, a high reverse voltage may be applied to specific light emitting cells in the bridge rectifier, such that the light emitting cells in the bridge rectifier may be damaged, thereby damaging the light emitting diode. Generally, when the reverse voltage applied to the light emitting cells exceeds 3 Vf, the possibility of damaging the light emitting cells may be increased. In order to prevent the light emitting cells from being damaged, the number of light emitting cells in the array of the light emitting cells connected to the bridge rectifier may be reduced. In this case, however, it may be difficult to provide a light emitting diode driven under a high-voltage AC power supply. Alternatively, the reverse voltage may be reduced by increasing the number of light emitting cells forming the bridge rectifier. However, this may degrade the utilization of the light emitting cells.